This invention relates to electron beam tubes of a type wherein an input signal having a fundamental frequency is applied to an electron beam to form electron bunches.
A klystron is a well known device in which velocity modulation of an electron beam is achieved following interaction with an applied high frequency input signal and a series of resonant cavities. FIG. 1 schematically illustrates a prior art klystron having an electron gun 1, an input resonant cavity 2, four intermediate cavities 3, 4, 5 and 6 and an output resonant cavity 7 followed by an electron beam collector 8. During operation, an electron beam is generated by the electron gun 1 along the axis Xxe2x80x94X of the klystron. A high frequency input signal, described as the fundamental frequency, is coupled into the input cavity 2 via a coupling loop 9 or other coupling means and causes an electric field to be produced across a drift tube gap 10 in the input cavity 2. This acts on the electrons arriving at the drift tube gap 10 to accelerate or decelerate them depending on their time of arrival with respect to the phase of the applied input signal. The resultant bunching of the electron beam is further enhanced by subsequent resonant cavities between the input cavity 2 and the output cavity 7. Three of these intermediate cavities 3, 5 and 6 (known as xe2x80x9cbuncher cavitiesxe2x80x9d) are tuned to a frequency which is slightly higher than the fundamental frequency, typically in the range of 1 to 5% higher, to give what is termed xe2x80x9cinductive tuningxe2x80x9d. The effect is to bring the electrons of the beam spatially closer together to produce tighter bunches and hence increase efficiency at the output cavity 7 from which an output signal is extracted via a coupling loop 11. The output cavity 7 is tuned to the fundamental frequency. In addition to the intermediate cavities tuned to just above the fundamental frequency, the resonant cavity 4 included near the input end of the device is tuned to slightly less than twice the fundamental frequency to provide what is termed xe2x80x9ccapacitive tuningxe2x80x9d. The capacitively tuned second harmonic resonant cavity 4 reduces the velocity spread of electrons in the bunches and hence improves efficiency at the output. It divides each electron bunch received from the intermediate cavity 3 into two bunches, each having a more uniform velocity distribution than the larger bunches from the intermediate cavity 3. The following inductively tuned intermediate cavities 5 and 6 act upon the divided bunches received from the second harmonic cavity 4 to bring them closer together, such that they are eventually recombined at the output cavity 7.
The present invention seeks to provide a device having improved efficiency. The invention is particularly applicable to klystrons but may also improve efficiency of other electron beam tubes employing density and/or velocity modulation in which bunching of electrons occurs during operation.
According to a first aspect of the invention, there is provided an electron beam tube of a type wherein an input signal having a fundamental frequency is applied to an electron beam to form electron bunches, the tube comprising: a buncher resonant cavity; a penultimate resonant cavity inductively tuned near a harmonic of the fundamental frequency; and an output resonant cavity from which an output signal is extracted.
Use of the invention enables improved efficiency to be achieved. The penultimate resonant cavity is tuned to give inductive tuning at a harmonic of the fundamental frequency, that is, it is tuned to a frequency which is slightly higher than the harmonic of the fundamental frequency, typically, 5% higher. This reduces the spatial spread of the bunches at the drift tube gap of the output cavity, making the bunches xe2x80x9csharperxe2x80x9d.
The input signal used to modulate the electron beam to form electron bunches may, for example, be a high frequency CW signal or may be modulated with, for example, a TV or other data signal. Although the invention is particularly applicable to klystrons, it may also be used with advantage in other types of tube in which electron bunching occurs such as for example inductive output tubes (IOTs) and tubes in which both density and velocity modulation of an electron beam takes place.
Preferably, there is included an input resonant cavity at which the input signal is applied. However, in some tubes, the input signal may be applied for example via a coaxial input line to directly modulate a grid located in front of a cathode of the electron beam gun, for example. Where an input cavity is included, preferably it is tuned to the fundamental frequency.
Preferably, the output cavity is tuned to the fundamental frequency. However, the invention may be employed in a frequency multiplier for example, in which case the output cavity may be tuned to a harmonic of the fundamental frequency.
In one advantageous embodiment of the invention, the penultimate resonant cavity is tuned to slightly greater than twice the fundamental frequency. However, the penultimate resonant cavity may be tuned to slightly above the third harmonic, fourth harmonic or other higher multiples of the fundamental frequency. It may be desirable to include one or more cavities immediately before the penultimate cavity each of which is inductively coupled at a harmonic of the fundamental frequency. The harmonic frequencies selected may be the same in each case or may be respective different harmonic frequencies. The harmonic frequency selected may be the same as that of the penultimate resonant cavity frequency.
The electron beam tube may also include a cavity tuned to slightly less than a harmonic frequency of the fundamental frequency to give capacitive tuning and hence reduce velocity spread of electrons in the bunches. Such a cavity is preferably located near the high frequency input of the tube.
In a particularly advantageous embodiment of the invention, the penultimate cavity includes a drift tube gap which is located at the position where an output cavity drift tube gap would be located if the penultimate cavity were not included in the tube. This geometry is particularly advantageous, giving good efficiency at the output cavity. In one preferred embodiment, the penultimate cavity is partially extensive within the volume defined by the output cavity. The penultimate and output cavities may have a common wall. In one preferred arrangement the penultimate cavity includes a conical wall extensive within the output cavity.
According to a second aspect of the invention, there is provided an electron beam tube of a type wherein a plurality of electron bunches are formed, the tube comprising: an output resonant cavity from which an output signal is extracted; and a penultimate resonant cavity inductively tuned near a harmonic of the fundamental frequency, the penultimate cavity being partially extensive within the output cavity.